GENESIS OF SILICIC PERALKALINE VOLCANIC-ROCKS IN AN OCEAN ISLAND SETTING BY CRUSTAL MELTING AND OPEN-SYSTEM PROCESSES - SOCORRO ISLAND, MEXICO

Silicic volcanic rocks from Socorro Island, Mexico, are dominantly por phyritic (alkali feldspar > sodic pyroxene +/- fayalite +/- Fe-Ti oxid es +/- aenigmatite) peralkaline trachytes and rhyolites. Typical of si licic peralkaline rocks, those from Socorro are enriched in Na2O, K2O, FeO, Zr and Nb, and depleted in Al2O3, Sr and Ba relative to silicic metaluminous rocks. The ranges of Nd and Pb isotope compositions are relatively restricted and similar to those of alkalic basalts from Soc orro Island. In contrast, Sr isotope compositions are more radiogenic than those of the alkalic basalts and trend toward the Sr-87/Sr-86 of modern seawater. Alkali feldspar-whole-rock pairs exhibit marked Sr is otope disequilibria. Gaps in SiO2 and K2O coupled with virtually ident ical high field strength element (e.g. Ar, Nb, Hf) and Th abundances b etween differentiated alkalic basalts and the least evolved peralkalin e trachytes are inconsistent with an origin for the trachytes by fract ional crystallization of the basalts; the most reasonable alternative is partial melting of mafic intrusive rock. Major and trace element co mpositions are consistent with peralkaline rhyolites being derived fro m parental trachytes by fractionation of an assemblage dominated by al kali feldspar; phase equilibria suggest that fractionation occurred in a shallow-level magma reservoir. At least two stages of open system p rocesses are implicated by Sr isotopic and trace element data: (1) int eraction between silicic magma and components of the shallow ocean cru st may be indicated by abundances of REE in excess of those predicted by fractional crystallization models and negative Ce anomalies; (2) po st-eruptive interaction between aqueous fluid and rock is suggested by whole-rock Sr isotope values that trend toward the Sr-87/Sr-86 of mod ern seawater, a positive correlation between whole-rock Sr-87/Sr-86 an d age, and reduction in whole-rock Sr-87/Sr-86 by acid leaching. The f luid apparently has Sr isotope characteristics comparable with those o f modern seawater and may be similar in composition to hydrothermal fl uids present at the summit of Socorro Island.